The present invention generally relates to an apparatus for dispensing viscous fluids and more specifically, to an electrically operated apparatus for dispensing viscous liquids, such as hot melt adhesives.
Pneumatic and electric viscous fluid dispensers have been developed for dispensing applications requiring precise placement of a viscous fluid. Pneumatic dispensers have a significant advantage in that the pneumatic solenoid operating the dispensing valve can be made very strong, so that the dispensing valve operation is essentially independent of the viscosity of the fluid being dispensed. However, pneumatic dispensers have disadvantages in that they generally have a shorter life than electric fluid dispensers, and the operation of the pneumatic solenoid is subject to less precise control than the electric solenoid in an electric fluid dispenser. Therefore, in some applications, electrically operated viscous fluid dispensers are preferred over pneumatic viscous fluid dispensers.
Generally, electrically operated dispensers include an electromagnetic coil surrounding an armature that is energized to produce an electromagnetic field with respect to a magnetic pole. The electromagnetic field is selectively controlled to open and close a dispensing valve by moving a valve stem connected to the armature. More specifically, the forces of magnetic attraction between the armature and the magnetic pole move the armature and valve toward the pole, thereby opening the dispensing valve. At the end of a dispensing cycle, the electromagnet is de-energized, and a return spring returns the armature and valve stem to their original positions, thereby closing the dispensing valve.
In the operation of an electric viscous fluid dispensing gun, the coupling between the coil and the armature is not efficient; and therefore, in order to achieve the highest actuation speed, a current pulse or spike is typically provided to the coil during an initial turn-on period in order to initiate the motion of the armature as quickly as possible. However, maintaining such a level of current to the coil quickly and substantially increases coil temperature. Further, maintaining such a high level of current increases the time required for the energy stored in the coil""s inductance to dissipate, thereby increasing the turn-off time and the time required to close the fluid dispenser. Therefore, after the initial current spike, the current through the coil is normally reduced to approximately the minimum value required to hold the armature in its open position by overcoming the opposing force of the return spring. Such a stepped current waveform is useful in reducing the current induced heat load in the coil, thereby allowing the coil to operate at a lower temperature than if the stepped waveform were not used. However, as is described below, the operation of the coil and armature during the fluid dispensing process creates other heat related issues that impact the quality of the fluid dispensing process.
The continued development and use of viscous fluid electric dispensers has resulted in more demanding performance specifications as well as a greater understanding of how heat in the dispenser can potentially effect performance. For example, the electric coil of an electric dispensing valve normally is not capable of providing the same forces as a pneumatic solenoid and therefore, is more subject to changes in resistance to valve stem motion that may be caused by changes in viscosity of the fluid being dispensed. Thus, as the viscosity of the fluid being dispensed changes, the load on the electromagnetic coil changes, and the time required to open and close the dispensing valve will likewise change. Such changes in timing of the dispensing valve opening and closing will change the location of the adhesive being dispensed on the substrate.
In addition to the above, newer applications have more demanding performance specifications and require ever-increasing gun speeds, that is, a shortening of the time required to open and close the dispensing valve. The operational speed of the dispensing valve can be increased by increasing the electrical power applied to the electric coil operating the valve. The electrical power is normally increased by increasing the current being supplied to the coil which also adds heat to the coil, thereby causing the temperature of the coil to rise. A hotter or higher coil temperature impacts the consistency of the viscous fluid dispensing in several ways. First, heat from the coil is conducted through the armature and the valve stem which is adjacent the valve seat and is surrounded by the viscous fluid. As the temperature of the armature fluctuates, for example, goes up, the viscosity of the fluid to be dispensed likewise fluctuates and, in this example, decreases, thereby changing the flow of the viscous fluid from the dispenser.
Second, the speed at which the armature can be moved between the open and closed positions is a function of the rate of change of current in the coil, which, in turn, is controlled by the electrical time constant of the coil. The electrical time constant is a function of the coil resistance which, in turn, is a function of temperature. The coil utilized in the viscous fluid dispenser discussed herein can experience an approximately 50% variation in resistance over its normal range of operating temperature. Such a change in resistance substantially affects the electrical time constant of the coil, thereby similarly affecting the speed at which the coil can open and close the valve.
The thermal time constant of the coil is a function of the coil mass and its thermal connections to surrounding materials such as the gun body and ambient temperature. The thermal time constant of the coil and its surrounding thermal system affects the time required for the thermal system to reach a steady state condition. When the dispensing system is running at a constant speed, and a steady state condition is achieved, the thermal time constant normally does not present a source of variation in the operation of the dispensing coil. However, the steady state condition can change for several reasons, for example, if the production line speed is either increased or decreased or, the dispensing gun is not operating and in the standby mode. Either of those conditions causes the coil temperature to change, and the thermal time constant presents a source of variations in the operation of the viscous fluid dispenser.
Of further concern is the maximum temperature rating of the coil wire insulation. Under normal operating conditions, the temperature rating of the wire insulation exceeds the wire temperature. However, in a worse case situation, if the temperature of the wire exceeds the temperature rating of the wire insulation, the integrity of the coil wire insulation may be compromised, thereby causing coil windings to short-circuit together. Any coil windings that short-circuit together will change the resistance of the coil and potentially adversely effect the consistency of the fluid dispensing operation of the dispenser.
Thus, by using a stepped current waveform, known electric fluid dispensers attempt to reduce the temperature of the coil. Further, it is known to utilize a heater in a manifold to which the fluid dispenser is mounted to control the temperature of the fluid circulating through the manifold and the fluid dispenser, thereby indirectly controlling the temperature of the dispenser itself. However, as will be appreciated, there have been no attempts to control the temperature of the fluid dispenser directly with a self contained device in order to maintain the electric fluid dispenser at a constant temperature.
The present invention provides an improved electric dispenser for viscous fluids that manages the thermal condition of the dispenser directly to provide a substantially improved, more consistent dispensing of viscous fluids. The electric dispenser of the present invention provides more consistent actuation of the dispensing valve independent of changes in the speed of operation of the dispenser. The electric fluid dispenser of the present invention reduces the range of temperature fluctuations resulting from changes in speed of the production line and changes in the frequency of operation of the fluid dispenser. Further, the electric fluid dispenser of the present invention maintains a generally constant coil temperature independent of the rate of gun operation. Providing a fluid dispenser that has a self-contained temperature control that reduces the range of temperature variations helps to maintain the viscosity of the fluid within the dispenser constant. By better controlling the temperature within the electric viscous fluid dispenser, a more consistent, faster and reliable operating cycle is achieved. Thus, the electric dispenser of the present invention provides the advantage of dispensing a viscous fluid more accurately, precisely and with a higher quality than was heretofore possible.
In accordance with the principles of the present invention and the described embodiments, the invention in one embodiment provides an electrically operated fluid dispenser for dispensing a viscous fluid onto a substrate during a run mode. The dispenser includes a body having an outlet and an armature disposed in the dispenser body for movement between an opened position allowing a fluid flow from the outlet and a closed position preventing the fluid flow from the outlet. A coil is mounted adjacent the armature and selectively generates an electromagnetic field for moving the armature between the opened and closed positions. A controller is connected to the coil and provides output signals to energize a coil positioned with respect to an armature within the fluid dispenser with a drive current to actuate the fluid dispenser and to simultaneously maintain the coil at an approximately constant temperature during the run mode.
In one aspect of the one embodiment, the controller includes power switches providing a drive current signal to the coil and a thermal controller providing a current waveform signal to the power switches. The current waveform signal operates the power switches to maintain the coil at a constant temperature in response to a temperature control loop.
In another aspect of the one embodiment, a heat transfer device is mounted in a heat transfer relationship with the dispenser body; and the controller is connected to the heat transfer device to cause the heat transfer device to selectively transfer heat between the heat transfer device and the dispenser body during the run and standby modes, thereby maintaining the dispenser body at a constant temperature during the run and standby modes.
In a second embodiment of the invention, the dispenser is turned off and does not dispense the viscous fluid during a standby mode of operation; and the controller provides further output signals to energize the coil with a current to maintain the coil at an approximately constant temperature during the standby mode.
In another embodiment of the invention, the coil has first and second windings disposed adjacent the armature, the controller selectively provides output signals to the first and second windings of the coil to cause current flow in the coil windings during the run and standby modes. The controller further includes a switching apparatus selectively placing the first and second windings in an additive relationship during the run mode to move the armature between the opened and closed positions and in an opposing relationship during the standby mode to maintain the armature immobile in the closed position.
In one aspect of this other embodiment, the controller includes power switches providing a drive current signal to the coil; and a thermal controller provides a current waveform signal to the power switches. The current waveform signal operates the power switches to maintain the coil at a constant temperature. The thermal controller generates the current waveform signal in response to changes in either power, current or temperature variables with respect to a respective desired value of those variables.
In another aspect of this other embodiment, the controller includes a high frequency power supply and a switching device connected between the power switches, the coil and the high frequency power supply. The switching device connects the coil to the power switches during the run mode and connects the coil to the high frequency power supply during the standby mode.
In a further aspect of this other embodiment, the controller includes power switches for connecting the coil windings in parallel across a power supply to permit the duty cycle of the current flow in each of the coil windings to be individually controlled, thereby uncoupling and independently controlling the power heating of the coil from the actuation power provided by the coil windings.
In a still further embodiment of the invention, a method is provided for operating an electric viscous fluid dispenser to maintain a coil positioned with respect to an armature within the dispensing gun at an approximately constant temperature during the run mode by heating the coil. In an additional embodiment, the above method includes maintaining the coil at an approximately constant temperature while the viscous fluid is not being distributed during a standby mode by heating the coil during the standby mode. In different aspects of this invention, the coil is heated during the run and standby modes by current flowing through the coil or by a separate heating and cooling heat transfer device. In a further aspect of the invention, the heating of the coil is controlled by an RMS value of the current in the coil.
The above embodiments of a fluid dispenser temperature controller have the advantages of reducing the range of temperature variations within the fluid dispenser and normally, maintaining the temperature of the fluid dispenser approximately constant. Thus, the fluid dispenser temperature controller does not rely on the user being able to control the best current waveform parameters, but instead, is adaptive and self-adjusting to maintain a constant coil temperature. The active temperature control protects the coil from overheating in the event that the user adjusts the current waveform such that an excessive temperature would otherwise be produced. With a constant coil temperature, the viscosity of the fluid within the dispensing gun is held more consistent, thereby improving the consistency of the dispensing process. Further, by maintaining the constant temperature over the full range of operating frequency of the dispensing gun, the coil temperature controller provides a further advantage of providing a higher quality and more consistent viscous fluid dispensing operation. In addition, such a temperature control permits the dispensing gun to be consistently operated at a rate that is very close to, if not at, the theoretical maximum temperature limit of the gun without overheating.
In a further embodiment of the invention, an electrically operated fluid dispenser has a body with a heater and a fluid passage intersecting first and second sides of the body and a dispensing outlet in fluid communication with the fluid passage. The dispenser includes a feed member having a fluid passage intersecting ends of the feed plate. One end of the feed member is mounted to the first side of the body with one end of the fluid passage in the feed member fluidly connecting with one end of the fluid passage in the body. The dispenser also has a cap mounted to the second side of the body to terminate the fluid passage on the second side of the body.
In one aspect of this further embodiment, the dispenser includes a second dispenser with a body having a heater, a fluid passage intersecting first and second sides of the second body and a dispensing outlet in fluid communication with the fluid passage. The first side of the second body is mounted to the second side of the first body with one end of the fluid passage in the second dispenser body fluidly connecting with an opposite end of the fluid passage in the first dispenser body.
In other aspects of this further embodiment, the dispenser includes a spacer plate disposed between the first and second bodies, and the heater is comprised of either a coil mounted with respect to an armature within the body or, a heating and cooling heat transfer device.
This further embodiment of the invention with the use of the coil heater has the advantage of maintaining the viscous fluid within the passage at the desired temperature without requiring a separate fluid distribution manifold plate to which the dispensing gun is normally mounted. A dispensing gun of this construction has the further advantage of being substantially more compact than the traditional manifold plate design. Further, the construction of the dispensing gun is substantially less expensive; and its simpler construction provides substantially greater flexibility in mounting the dispensing gun with associated equipment.
In yet another embodiment of the invention, a temperature monitor for monitoring a temperature of an electrically operated fluid dispenser has a coil mounted adjacent an armature within the dispenser, the coil selectively generates an electromagnetic field to move the armature between opened and closed positions. The temperature monitor includes current measuring apparatus for measuring a current in the coil and a comparator for comparing a measured current value to a desired current value. An indicator provides an indication representing a relationship between the measured current value and the desired current value.
In different aspects of this embodiment, the temperature monitor measures the RMS value of the current in the coil and has different indicators for providing different indications representing different values of the measured current relative to a desired current value.
The thermal monitor has the advantage of providing the user with a real time indication of whether the user""s adjustments to the current waveform provide a coil temperature that is less than, close to or in excess of the maximum coil temperature. In addition, the thermal monitor has the further advantage of helping the user select the temperature limits which are appropriate for the dispensing gun being used and the dispensing application being effected.
Various additional advantages, objects and features of the invention will become more readily apparent to those of ordinary skill in the art upon consideration of the following detailed description of the presently preferred embodiments taken in conjunction with the accompanying drawings.